Koalas have a
comparatively short
gestation period of 34-36 days. They usually have only one baby (known
as a joey) per year. Their newborn are 3/4 inch (1.9 cm.) long and
less than 1 gram in weight. They appear to be still at an immature
fetal stage. They are pink, hairless, blind, and deaf (they have no
ears at this time). However, they have a good sense of smell, a sense
of direction, and strong hands which they use to crawl unaided from the
birth canal to their mother's abdominal downward facing pouch. There
are two nipples inside the pouch. The joey usually remains more or
less anchored to a nipple for half of a year. Milk is the only food
during this period. The nipple swells in the joey's mouth, which helps
it remain anchored there. The mother also contracts the opening of her
pouch to prevent her baby from falling out.

By 6 months, joeys
are fist size and look like an adult in their proportions. They now
have fur as well as functioning eyes and ears. They periodically leave
the pouch beginning at this time to explore their immediate surroundings but return to
hide and sleep. They also transition to being carried on their
mother's back. Their teeth are starting to erupt, which allows them
to gradually switch to a diet of eucalyptus leaves. This begins by the joey eating the mother's "pap" which comes from her cecum (a small pouch
between the small intestine and the colon). This soft, runny "pap" is
a specialized form of protein rich feces containing bacteria that allow
koalas to digest what would otherwise be toxic eucalyptus leaves. The
bacteria become permanent residents in the joey's digestive system.
When weaning is complete by around 12 months, the mother leaves her joey to
fend for itself.